The present invention relates to a charging apparatus and a charging method for charging batteries of, for example, electric vehicles. More particularly, the present invention pertains to a charging apparatus and charging method that prevent an abnormal increase of voltage when charging a battery.
FIG. 4 is an electric circuit diagram of a known charging apparatus for electric vehicles. The charging apparatus includes a direct current power source E, an inverter 11, a transformer T, and a rectifier circuit 12. The inverter 11 converts a direct current from the power source E to an alternating current. The transformer T includes a first coil T1 and a second coil T2. When a direct current is applied to the first coil T1 from the inverter 11, an induced electromotive force is generated in the second coil T2. The rectifier circuit 12 rectifies the induced electromotive force, which is an alternating voltage, to a direct current and then delivers the direct current to a battery B installed in the electric vehicle (not shown). As a result, the battery B is charged.
The direct current power source E, the inverter 11, and the first coil T1 function as a first circuit and are provided, for example, in a charge control box (not shown) located on the ground. The rectifier circuit 12 and the second coil T2 function as a second circuit and are provided in the electric vehicle. The control box includes a coupler having the first coil T1, and the electric vehicle includes a coupler having the second coil T2. The couplers are detachably coupled. When the couplers are coupled, the first coil T1 faces the second coil T2 in a contactless state. The battery B is charged in this state. Since charging is performed through the coils T1, T2, which are not in contact, the above described charging apparatus is called a non-contact charging apparatus.
It is generally known that the lower the frequency of alternating current generated by the inverter 11 is, the greater the power of the direct current produced in the rectifier circuit 12 is. Accordingly, it is necessary to vary the frequency of the alternating current generated in the inverter 11 in a wide range so that the value of the direct current power delivered to the battery B is varied over a wide range. To achieve this, it is necessary to control the ON/OFF cycle of switching transistors in the inverter 11 over a wide range. This increases the burden on the control apparatus controlling the inverter 11. Also, to handle alternating currents having a wide range of frequencies, a large transformer T must be used.
To solve this problem, as shown by dotted lines in FIG. 4, a capacitor C is connected between the terminals of the second coil T2. The capacitor C functions as a booster circuit that increases the induced electromotive force generated in the second coil T2. Also, the capacitor C and a reactor L are serially connected to the first coil T1 and form a resonance circuit. The resonance circuit is capable of widely varying the direct current voltage obtained by the rectifier circuit 12 by simply varying the frequency of alternating current generated by the inverter 11 in a narrow range, more specifically, in a low frequency range.
Japanese Unexamined Patent Publication No. 2-303329 describes a charging apparatus including a circuit breaker in the second circuit. The circuit breaker disconnects the rectifier circuit and the battery when an excessive amount of current flows from the rectifier circuit to the battery. In the conventional apparatus shown in FIG. 4, also, a fuse F that serves as a circuit breaker is provided between the rectifier circuit 12 and the battery B. Further, a relay (not shown) is serially connected to the battery B. If the fuse F or the relay is cut off during charging, the output voltage of the rectifier circuit increases immediately (in about 100 microseconds). This is because the output voltage of the rectifier circuit 12 is increased when the induced electromotive force generated in the second coil T2 is increased in the capacitor C.
Suppose, for example, the ratio between the number of turns of the first coil T1 and the number of turns of the second coil T2 is one to one, the alternating voltage generated in the inverter 11 is 400V, and the output voltage of the rectifier circuit 12 is 420V to 430V. When the fuse F is cut off in this case, the output voltage of the rectifier circuit 12 instantaneously goes up to 800V to 900V. The high voltage may be beyond the voltage resistance of the elements constituting the second circuit, that is, a diode in the rectifier circuit 12 or a smoothing capacitor C5 that is located between the rectifier circuit 12 and the battery B.
Japanese Unexamined Publication No. 7-39077 discloses a charging apparatus that sends the operation condition of the second circuit to the first circuit by optical communication and that controls the operation of the first circuit in accordance with the operation condition of the second circuit. This kind of charging apparatus permits stopping the operation of the first circuit when the fuse F is cut off in the second circuit. However, the operation of the first circuit is stopped in response to the cutoff of the fuse F, and this is too slow to prevent an abnormal increase of voltage in the second circuit.
It is possible to provide a switch for cutting off the rectifier circuit 12 from the second coil T2 when a high voltage is generated by the cutoff of the fuse F. However, even using the switch, a high voltage is applied to the capacitor C that is a booster circuit.